Beacon: bentonite mechanical evolution

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The aim of Beacon is to develop the understanding of fundamental processes that lead to material homogenisation, as well as to improve capabilities for numerical modelling. In earlier assessments of bentonite EBS, the mechanical interaction between the installed bentonite components has been neglected and an “ideal” ﬁnal state has generally been assumed.

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EPJ Nuclear Sci. Technol. 6, 23 (2020) Nuclear Sciences © P. Sellin et al., published by EDP Sciences, 2020 & Technologies https://doi.org/10.1051/epjn/2019045 Available online at: https://www.epj-n.org REVIEW ARTICLE Beacon: bentonite mechanical evolution Patrik Sellin1,*, Mary Westermark1, Olivier Leupin2, Simon Norris3, Antonio Gens4, Klaus Wieczorek5, Jean Talandier6, and Johan Swahn7 1 Svensk Kärnbränslehantering AB (SKB), Evenemangsgatan 13, Box 3091, 169 03 Solna, Sweden 2 Nagra, Hardstrasse 73, Postfach 280, 5430 Wettingen, Switzerland 3 Radioactive Waste Management, Building 587, Curie Avenue, Harwell Oxford, Didcot, Oxfordshire OX11 0RH, UK 4 Department of Civil and Environmental Engineering, Universitat Politècnica de Catalunya (UPC) BarcelonaTech, Jordi Girona 1-3, Ediﬁci D-2, 08034 Barcelona, Spain 5 Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH, Abteilung Standortauswahl, Bereich Endlagerung, Theodor-Heuss-Str. 4, 38122 Braunschweig, Germany 6 Andra, 1/7, rue Jean Monnet, Parc de la Croix-Blanche, 92298 Châtenay-Malabry Cedex, France 7 Miljöorganisationernas kärnavfallsgranskning (MKG), Första Långgatan 18, 413 28 Göteborg, Sweden Received: 12 March 2019 / Accepted: 16 September 2019 Abstract. The aim of Beacon is to develop the understanding of fundamental processes that lead to material homogenisation, as well as to improve capabilities for numerical modelling. In earlier assessments of bentonite EBS, the mechanical interaction between the installed bentonite components has been neglected and an “ideal” ﬁnal state has generally been assumed. Key features of the project are (1) re-evaluation of the available knowledge to extract the crucial data to compile the qualitative and quantitative data and to enhance the conceptual understanding. (2) Enhanced, robust and practical numerical tools based on a good scientiﬁc understanding, which have the expected predictive capabilities regarding the evolution of engineered barriers and seals. (3) A developed database with experimental data needed by the quantitative models. (4) Veriﬁed calculation tools based on experimental results in different scales. The Beacon project is required for the pan- European objectives at building conﬁdence amongst regulators and stakeholders regarding the performance of the engineered barriers in a geological repository. 1 Introduction long-term performance refers to the period for barrier saturation and evolution of the hydro-mechanical state, The key objective of Beacon [1] is to reﬁne and verify the which could range from 10 to 1000s of years. In current and tools needed for the assessment of the mechanical evolution future applications for repositories, the regulators will of installed bentonite components and the eventual expect the applicants to have a sufﬁcient predictive performance of the barrier. The aim is to demonstrate capability of the barrier evolution from the installed to that the performance of present designs for buffers, the ﬁnal state. backﬁlls, seals and plugs is appropriate. In addition, for Beacon is focused on the direct application to real repository designs particularly in crystalline host rock, the assessment cases in actual repository systems. A few cases results may also be used for the evaluation of effects of mass from relevant repository systems have therefore been loss from a bentonite barrier in long term perspective. selected as test examples. The systems intended to be The driver for this project is repository safety, and the evaluated in Beacon include three cases: (1) a tunnel plug demands of waste management organizations to verify that based on the ANDRA design, (2) a disposal cell from the the material selection and initial state design fulﬁl the long- Nagra concept, (3) the KBS-3 deposition tunnel backﬁll. term performance expectations. For this project, the initial These are representative of the primary areas of uncertain- state refers to the period of installation of the barrier, while ty in density homogeneity. These examples cover a broad range of issues and the results should be applicable to other concepts and systems as well. The cases are illustrated in * e-mail: patrik.sellin@skb.se Figures 1–3. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
2 P. Sellin et al.: EPJ Nuclear Sci. Technol. 6, 23 (2020) different from the ﬁnal state after full saturation. It is therefore crucial to consider: – the mechanical evolution during the saturation phase; – the ﬁnal state at equilibrium. A ﬁrm understanding of the mechanical evolution is necessary to ensure that a given design will meet the expected performance targets. The scientiﬁc-technical work in Beacon is structured in ﬁve work packages (WP1-5), dissemination and training is handled in two work packages (WP6-7) while coordination Fig. 1. Tunnel plug in the ANDRA concept. and project management is covered in one single work package (WP8). The interconnections between the work packages are illustrated in Figure 4. WP1 is the main driver for the entire project. The waste management programs involved are all represented in WP1, through the implementer or equivalent organiza- tion. The objective of WP1 is to deﬁne the important issues concerning the mechanical properties of bentonite and to deﬁne how these should be treated. In WP2 the existing knowledge base is treated. The key objective of WP2 is the sharing of knowledge and experience. The partners have distributes information and results from earlier assessments, design considerations, experiments and modelling tasks. A strong driver for a joint project is the current limitations in the predictive capability in the numerical models. The issue of homogenisation and swelling is Fig. 2. Disposal cell in the Nagra concept. challenging both from a conceptual and a numerical point of view. The purpose of WP3 is to identify and resolve the shortcomings of current models. Although there is a substantial experimental database available for the project, it is necessary to perform additional experiments to support the model development in WP3 and the model testing in WP5. The experimental work is coordinated in WP4. WP4 consists of experi- enced experimental groups, which have the ﬂexibility to adapt the experimental work to support the needs of WP3 and WP5. The core component of Beacon is WP5. The main effort is performed in this work package. The overall objective of WP5 is to simulate the assessment cases deﬁned by WP1. In order to do this, the available models have to be tested ﬁrst on results from laboratory experi- Fig. 3. KBS-3 Tunnel backﬁll. ments and later on results from large scale ﬁeld tests to gain conﬁdence in their predictive capability. The next step is to actually test the predictive capability, by means of “blind” predictions of experimental results. Finally, the models will Beacon builds upon experience by waste management be used to evaluate the assessment cases. organizations from different countries and technology providers over the past 30+ years. Great gains have been made in understanding individual bentonite components through experimental and modelling work. Yet short- 2 WP1 deﬁnition of assessment case/ comings in the state-of-the-art knowledge exist which still application to the assessment cases inhibit conﬁdence toward repository operation. The sealing ability is crucial for bentonite barriers in all The needs in safety assessments regarding the evaluation geological repository concepts. Sealing is achieved by the of heterogonous backﬁll properties are addressed, in combination of a high swelling pressure and a low hydraulic particular to what extent heterogonous material property conductivity. The swelling pressure will impact the other distributions will comply with performance targets are barriers in the repository as well. The mechanical addressed in WP1. The outcome of this work package properties of the installed EBS, that will consist of a is expected to be a (hydro)-mechanical assessment combination of blocks, pellets and voids, will be completely of the selected assessment case, considering a range of
P. Sellin et al.: EPJ Nuclear Sci. Technol. 6, 23 (2020) 3 Fig. 4. Interconnections between the work packages in Beacon. uncertainties in the boundary conditions that would density, swelling pressure and hydraulic conductivity as eventually result in a set of requirements under consider- relevant natural properties for the bentonite regarding ation of the host rock and the repository design. heterogeneity, while organic carbon or thermal conductivi- The ﬁrst deliverable was a report compiled with the ty seem to be incidental to the homogenization process. answers to a questionnaire that was distributed to the All participating waste management organizations different WMOs or their representatives. The question- agree that the most valuable output from Beacon would naire aimed at reﬂecting the state-of-the-art regarding the be material models that are accurate enough to be used as a treatment of heterogeneous bentonite density distribution tool for design and engineering purposes, i.e. to assess the and properties in the safety case. The questionnaire behaviour and performance of the bentonite-based EBS consisted of three different parts: (1) application of both on the short- and long-term under variable design and bentonite in the speciﬁc design; (2) the required perfor- environmental conditions. It is expected that, if prepara- mance of bentonite; (3) detailed characterization of the tion of the sealing material (e.g. pellets) and emplacement required properties of the bentonite. method are performed properly, heterogeneity will not be The conclusions from the answers to the questionnaire problematic for safety cases and the buffer material can be were that occurrence of heterogeneity in the repositories represented in the safety assessment by a well-chosen could impact the safety functions of bentonite components. homogeneous material. Therefore, it needs to be determined to what extent this could affect the safety case of the repositories. Heterogene- ity can occur in the initial material, through the 3 WP2 collection and compilation of existing emplacement or the re-saturation phase as well as on the data and available models long term after re-saturation of all repository components. The heterogeneities in the initial state, i.e. after installation The most important outcome of WP2 so far has been a of the EBS, are mainly due to density differences. report that documents the information that has been made Inhomogeneous saturation and swelling of bentonite could available to the Beacon project by the project partners cause irreversible damage. The role of uncertainties related and associated organisations. The information relates to to these bentonite heterogeneities is addressed in most experiments at a number of different scales as well as repository concepts using a deterministic approach, deﬁned modelling studies that have been undertaken. The report with a preferred density value. There are several natural utilizes information provided by Beacon partners regarding properties of bentonite that may impact the degree of experiments that have been carried out in earlier projects, homogenization. Most waste management organizations with the purpose to build a database of experiments which consider water content, original exchangeable cations, bulk can be used during the Beacon project and even beyond.
4 P. Sellin et al.: EPJ Nuclear Sci. Technol. 6, 23 (2020) The report documents the available data but does have the purpose to propose any experiments for consideration within the Beacon project. This objective will be handled by the Work Package leaders, based on the information Suction contained in the report. The report provides clear referencing to the underlying reports which contain further information, to facilitate the process of selecting datasets for modelling within Beacon. The process for collecting and compiling information into the database involved: – creating a designated data form to collect appropriate information; – requesting that Beacon partners and associated organ- isations ﬁll out the form for any studies that could be Stress relevant to Beacon; – collation of the completed data forms into a database; Fig. 5. Suggested suction-stress paths to check stress path – arranging a workshop for the discussion of the database dependency from an unsaturated to a saturated state. Stress may and deﬁnition of additional ﬁelds that would aid selection be vertical total stress (in oedometer tests) or mean total stress (in of experiments for modelling cases within Beacon; isotropic loading tests). – request for additional information to complete the new ﬁelds in database; laboratory and ﬁeld-scale tests. Ideally, those constitutive – completion of the database. models should consider the following cases: The data forms entered into the database have been – saturated and unsaturated materials; divided in three categories: laboratory experiments, – compacted bentonite (Blocks) and granular bentonite mock-up experiments and in situ experiments. The (e.g. pellet-based); laboratory experiments include both experiments with – isothermal and non-isothermal conditions, the purpose to measure material properties, as well as although it is recognised that not all models will necessarily experiments that simulate repository conditions in a have this comprehensive level of generality. smaller scale. There are many experiments listed in the To facilitate a more common assessment of the models, database, at a range of scales (from bench top laboratory the teams were asked explicitly what the model capabilities experiments to full scale ﬁeld experiments), a small were concerning a number of features of behaviour: number of these were designed speciﬁcally for studies of – dependence of swelling strain on applied stress and on dry bentonite homogenisation, but the majority were density; originally designed for other purposes. Nevertheless, all – irreversibility of strains in wetting/drying cycles; these experiments provide a valuable source of informa- – behaviour during swelling stress test. Dependence of tion on the mechanical properties of bentonite and the swelling pressure on dry density; mechanical evolution of a bentonite barrier within a – stress path dependence from an unsaturated to a repository. The database contains information on the saturated state (Fig. 5); type of bentonite considered in the tests, the boundary – stress path dependence from a saturated to an unsatu- conditions and initial and ﬁnal heterogeneities within the rated state; experiments, but also the range of measurements taken – dependence of strains developed in a temperature cycle in the experiment. This will give Beacon partners and (increase/decrease) on OCR (Overconsolidation Ratio) especially Work Package leaders the opportunity to (or stress). interrogate the database and ﬁnd experiments of interest for consideration as case studies within the Beacon The ﬁrst ﬁve items correspond to an isothermal project. Furthermore, knowledge about experiments that formulation whereas the sixth one requires the incorpo- have been undertaken previously will guide decisions on ration of temperature effects. The features selected for this new experiments to be undertaken. purpose are those that are deemed, in principle, most relevant to explain the evolution of engineered barriers and 4 WP 3 model development seals during the transient phase. The model capabilities at the beginning of the project are available as deliverable 3.1. Work Package 3 plays a central role in the structure of the project as it is devoted to the development of the constitutive models for describing the hydro-mechanical 5 WP4 lab testing behaviour of the bentonite in an appropriate manner. It is recognized that current models face limitations in their The objectives of the Beacon experimental studies are to predictive capabilities and signiﬁcant advances are re- provide input data and parameters for development and quired. The models must prove their predictive capabili- validation of models and to reduce uncertainties about ties, reliability and robustness and they should preferably conditions and phenomena inﬂuencing bentonite homoge- be grounded on a good understanding of the phenomena nisation. Both the homogenisation of an initially inhomo- involved. To this end, they will be validated using several geneous bentonite system and the persistence or
P. Sellin et al.: EPJ Nuclear Sci. Technol. 6, 23 (2020) 5 Fig. 6. Isochoric cell for hydration tests on bentonite block/pellets and swelling pressure evolution in two tests. development of inhomogeneities in the bentonite systems positions. The ﬁnal appearance of the clay was homoge- under various mechanical and hydraulic conditions are neous, with no discernible pellets. There was a slight investigated. Eight experiment teams perform tests decrease of water content from the hydration surface to involving different bentonite materials and hydraulic the top of the sample (from 35% to 30%), while the dry and mechanical boundary conditions. As an example, density increased in this sense (from 1.39 to 1.46 g/cm3). CIEMAT’s hydration tests in large-scale isochoric cells are The average dry density of the bentonite block consider- presented here. ably decreased as a consequence of saturation and that of The work focuses on the conceptual understanding of the pellets part increased. The degree of saturation was the evolution of bentonite fabric and microstructure upon close to 100% in every position. The pore size distribution hydration and the factors affecting them. To accomplish signiﬁcantly changed with respect to the original samples: this, hydration tests are being performed in isochoric cells the macroporosity of the pellets decreased, whereas the (Fig. 6) to analyse the fabric and microstructure evolution microporosity of both the pellets and of the block of initially inhomogeneous FEBEX bentonite. In the tests increased. half of the sample was composed of bentonite pellets (initial dry density 1.3 g/cm3) and the other half of a bentonite block (initial dry density 1.6 g/ cm3). The two 6 WP5 testing veriﬁcation and validation samples were prepared in the same way, but one of them of models was hydrated under a constant ﬂow rate of 0.05 cm3/h (MGR22) and the other one under a constant injection One of the objectives of Beacon project is to improve pressure of 14 kPa (MGR23). The ﬁrst boundary condi- models to simulate bentonite component evolution along tion tried to simulate a host rock with limited water the repository life. In this context, a speciﬁc work package availability, whereas the other one simulated a repository is dedicated to veriﬁcation and validation of models. with plenty of water in which the water intake is Based on an inventory performed within the project controlled by the bentonite permeability. The axial identifying experiments in link with mechanical evolution pressure development in the two tests is shown in Figure 6. of repository bentonite barrier materials, a selection of test There was a signiﬁcant difference in swelling development cases were made. The test cases are based on experiments kinetics, but although it took much longer to reach an performed at several scales: from lab tests (cm) to ﬁeld tests equilibrium value for the test performed under low water (scale 1) which are relevant in regards of the Beacon inﬂow (MGR22), the ﬁnal swelling pressure value, once objectives. the samples were saturated, was the same in the two cases, Partners involved in WP5 make simulations on the about 3 MPa, which is the expected value for a granular chosen test cases and produce speciﬁed results. The FEBEX sample compacted to the average dry density of objective of those simulations is not only to reproduce the block/pellets set (1.42 g/ cm3). the experimental results which most of the time could be Once the sample from test MGR22 was dismantled, its done by identifying some relevant parameters but also to physical state was checked by determining the water detect where the difﬁculties in terms of modelling are. The content, dry density and pore size distribution at different aim is to be able to improve our capacities of prediction of
6 P. Sellin et al.: EPJ Nuclear Sci. Technol. 6, 23 (2020) long-term behaviour for bentonite based components. The exercise shows the difﬁculties of the models to Comparisons of results and analyses of the differences will represent the transient phase that is constituted mainly lead to an improvement of the physical and numerical by water saturation and development of swelling models creating a strong link with WP3 (model develop- pressure. ment), and should give a feedback to the experimentalists At the top of the sample, dry density and water content involved in the dedicated work package (WP4). at the end of the test are higher due to the introduced void The strategy was to select tests at laboratory scale during the test. The water content obtained at the end of where homogenization processes have been highlighted, the test when the sample was dismantled is compared to which will constitute elementary bricks to tackle bentonite the numerical results (Fig. 9) showing that the general evolution modelling at a larger scale. The selected tests are: trend for water content is well reproduced. Models seem – swelling pressure tests for compacted plugs with free able to reproduce the ﬁnal heterogeneity of properties volume available TEST B1.7 from Clay Technology AB; observed at the end of the test. – swelling pressure tests for pellets mixture TEST B1.16 from CEA, Andra; – swelling pressure tests for block and pellets structure TEST B1.6 from Posiva. The ﬁrst test for modelling during this exercise was a MX-80 bentonite with an initial dry density of 1655 kg/m3. The test had two successive phases (Fig. 7). The ﬁrst phase was a classical swelling pressure test at constant volume. At stable swelling pressure, the upper piston was released and ﬁxed at a higher position creating a certain volume for expansion. A second stage of swelling started when the expanding bentonite had the new volume. At equilibrium, i.e. when no or negligibly small changes are seen in the recorded swelling pressure, the sample is dismantled and sliced axially to determine the water content and distribution of density in the direction of expansion. Fig. 7. Schematic of the axial swelling tests. The red lines show Several partners modelled the test obtaining results the lubricated walls and the blue lines indicate ﬁlters and water in good agreement with the measurements at the end of supply. The radial pressure transducer is located 10 mm above the the ﬁrst stage for the axial swelling pressure (Fig. 8). bottom of the specimen. Fig. 8. Axial pressure evolution comparison between the experiment and the models.
P. Sellin et al.: EPJ Nuclear Sci. Technol. 6, 23 (2020) 7 Fig. 9. Water content at the end of the test comparison between the experiment and the models. The ﬁrst analysis was very useful to identify some where there are consultation processes as a part speciﬁc points that need to be investigated and on what the repository inplementation. model evolutions should be focused. The second test in the The Beacon project has arranged one speciﬁc training list above, performed on pellets, highlighted the same kind course during the project, within WP7. The course took place of conclusions. The hydration phase is always difﬁcult to at the Universitat Politècnica de Catalunya (Spain).The capture with the models. course aimed to give an overview of the current approaches The next task will be to model large scale ﬁeld tests to and capabilities concerning the constitutive and numerical show the capacity of the models to reproduce in situ modelling of the hydro-mechanical behaviour of bentonite experiments. Three experiments have been selected: EB and other swelling clays. The context of the course was the Engineered Barrier Emplacement Experiment (Mont ﬁeld of nuclear waste management; however, the concepts Terri), FEBEX Full-scale Engineered Barrier Experi- and methods that were presented have a much wider scope of ment in Crystalline Host Rock (Grimsel), CRT Canister application. The topics of the course were: Retrieval Test (Äspö). – the fundamental science behind the mechanical and hydraulic properties of bentonite; – current constitutive modelling approaches; 7 Civil society interaction, dissemination – numerical modelling and examples of application; and coordination – the issues around the mechanical evolution of bentonite in nuclear waste management; The overall aim of WP6 is involve civil into the research – hands-on training with a computer code. performed in the Beacon project. Representatives from civil society are taking part in project and work package The course had 36 participants and addressed the meetings to gain an inside information about the nuclear waste management community as well as to results. The aim of the work package is to facilitate students in areas of soil and material science and civil, the interpretation of scientiﬁc results and other output environmental and mechanical engineering. from WP1–5 to the interested public and enable for civil society local and national representatives to interpret, 8 Conclusion discuss and elaborate on the research results and other information generated by the project. This will prepare BEACON is now about half way into the project. So far the civil society for active participation in future situations work has been both focused and according to plan. The
8 P. Sellin et al.: EPJ Nuclear Sci. Technol. 6, 23 (2020) State of Art reports from WP2 and WP3 are important The BEACON project receives funding from the Euratom summaries of the knowledge that never has been collected research and training programme 2014-2018 under grant in a similar manner before. They will both be an important agreement No 745942. foundation for current and future mechanical assessments of bentonite. The benchmark tests in WP5 have also been very successful: they have pointed out that there are real challenges, but also have shown that there are model Reference concepts available that should be able to meet those challenges. 1. http://www.beacon-h2020.eu Cite this article as: Patrik Sellin, Mary Westermark, Olivier Leupin, Simon Norris, Antonio Gens, Klaus Wieczorek, Jean Talandier, Johan Swahn, Beacon: bentonite mechanical evolution, EPJ Nuclear Sci. Technol. 6, 23 (2020)